1. Field of the Invention
The present invention relates to binders comprising urea-formaldehyde resins which contain high levels of buffering salts and use of the buffering salts to speed cure of the urea-formaldehyde resins.
2. Background Discussion
A buffered system is one in which little or no change in pH occurs with the addition of small quantities of an acid or base. When added to a system, buffer-inducing materials (e.g., potassium phosphates, sodium acetate), tend to buffer systems over a fairly specific pH range. The pH range over which the system is buffered will vary, depending on the type of buffering material employed. Some buffer-inducing materials buffer over more than one specific pH range. For example, potassium phosphates buffer over the pH ranges 1 to 3, 6 to 8 and 11 to 13. Thus, when dipotassium phosphate is added to a UF resin, the resin exhibits a relatively strong resistance to a change in pH with addition of acid or base in the pH ranges from about 1 to 3, 6 to 8 and 11 to 13. Outside of these ranges of buffering action, the pH can be changed with the addition of small quantities of acid or base. Other salts are considered as being "neutral." These types of salts do not induce a buffer when added to aqueous systems. Examples of these are sodium chloride, sodium sulfate, potassium chloride, and magnesium chloride. In general, neutral salts are made from the combination of a strong acid with a strong base, whereas buffering salts are made from the combination of a weak acid and a strong base, or alternately, a weak base and a strong acid.
Urea-formaldehyde (UF) adhesive resins are employed in a wide range of wood bonding systems. These include hot-press particleboard, medium-density fiberboard, and plywood applications, among others. Urea-formaldehyde resins are often referred to as thermosets, which rely upon heat to obtain adequate cure in a reasonable period of time. It is well understood that UFs may be cured at ambient temperatures by catalysis with free acid. In practical uses, a combination of a moderate increase in acidity and elevated temperatures is generally employed to achieve cure. To make particleboard, it is common to rely upon the acidity inherent in the wood furnish to provide reduced pH for cure, the pH normally varying from about pH 4-6.5, depending on the wood species. UF resins are only storage stable at pH's above 7.0. Thus, resins generally contain small, e.g., 0 to 0.3 weight percent, of buffer so that minimal resistance to a drop in pH is exhibited by the resin when sprayed onto and intimately mixed with the wood furnish. Alternately, latent catalysts, or free acid itself, may be added if faster cure speeds are required such as in core-layers which heat more slowly in comparison to surfaces, or when using high pH woods, or in particular for plywood applications wherein intimate mixing of the resin and wood is not achieved and therefore minimal pH drop is experienced by the resin. Latent catalysts commonly employed include amine-acid salts, such as NH.sub.4 Cl and (NH.sub.4).sub.2 SO.sub.4, which react with free formaldehyde generated during cure, and subsequently release free acid. Other non-buffering inorganic salts are also used to enhance cure speed. Examples of these include MgCI.sub.2, MgSO.sub.4, NaCl and Na.sub.2 SO.sub.4. The exact mechanism by which these salts enhance cure speed in UF adhesive systems is not presently known, and may vary depending upon the salt.
As mentioned, except in certain circumstances, particleboard resins are generally made so that they exhibit a low degree of resistance to a drop in pH, i.e., they are generally low buffered so that with intimate mixing of the resin with wood surfaces, a sufficient drop in pH for speedy cure of the glue will occur. Usually, only a minimal amount of buffering is employed to enhance storage life of the UF resin. This small amount of buffering prevents the pH of the resin from dropping below about 7.5 during storage. There are instances in which buffering salts are employed so that cure of the resin is prevented until such time that the formed mat of resin plus wood reaches the hot press. This is at times necessary because, after blending the resin with wood furnish, a significant amount of time may pass before the resin-treated furnish mixture reaches the pressing station. Since the wood furnish is almost always warm due to previous drying procedures, and many times the manufacturing plant itself is very warm, the acidity of the resin may increase and begin curing before consolidation for the mat in the press, which can result in reduced board properties. Therefore, it has been the practice to employ the use of buffering agents to slow the cure of the resin by retarding the drop in pH in cases wherein precure may occur as described above. It is commonly believed and accepted that buffering-type salts included in particleboard resins will only slow their cure. Thus, only the least amount of buffering salts necessary to prevent precure are employed. In plywood applications, buffers are again usually kept to a minimum because latent catalysts must be employed to overcome the buffer in order to reduce the adhesive pH significantly below pH 7.0 to promote cure.
Generally speaking, it is an advantage to impart faster cure to UF resins. The time required during the pressing stage is usually the deciding factor which limits the total possible production in most wood composite panel manufacturing processes. Therefore, any catalyst which can speed cure, i.e., which will impart improved performance properties at shorter press times, is desired. Shortening the press time by only a few seconds can result in considerable increases in profits to board manufacturers.
An increasing problem in the use of UF resins faced by panel producers during the past 10 to 15 years has been the requirement for lower formaldehyde-emitting panel products. To meet this demand for lower fuming products, resin producers have moved to resins with lower F/U molar ratios. In general, lower-fuming resins result in reduced board properties, due to a lower extent of cure. Also, the resins tend to be slower curing than the higher mole ratio, higher fuming, more reactive resins of the past. Because of this, new catalyst systems which might improve board properties (especially at short press times), while maintaining equivalent formaldehyde emission potential, would have a large economic benefit for board producers. Equally, board manufacturer's total plant emissions of formaldehyde and other volatile organic compounds (VOCs) which arise during pressing, board cooling, and at other locations in the various manufacturing steps have recently become an important factor that may limit plant production. It is critical that board manufacturers are able to either reduce stack emissions by using lower fuming resins, or by increasing production (with the same total stack emission level) by employing faster curing resins which exhibit equivalent or lower formaldehyde fuming potential.
The book by B. Meyer, "Urea-Formaldehyde Resins" (Addison-Wesley Publishing Company, 1979) reviews UF resin chemistry patent and journal literature from the turn of the century up to about 1979. Meyer's book covers the use of UF resins in the manufacture of cast polymer glasses and as wood adhesives. Meyer points out that the traditional use of buffers in UF resins has been that of pH control. On page 112 he notes that in cold set wood adhesive applications where high acid contents are required for cure, buffers may be used to prevent the pH of the acid catalyzed system from going too low. Overly low pH would result in degradation of the glued joint. On page 169 he points out that all particleboard UF resins are buffered to some degree, but the level may be controlled to prevent pH changes when necessary or advantageous. U.S. Pat. No. 3,335,113 to Dundon (column 2, lines 45-55) indicates the same type of considerations must be made in the use of UF resin for textiles. U.S. Pat. No. 1,460,606 to Ripper points out the retarding of cure and the rate of reactions by buffering agents.
In the very early use of UF resins for manufacture of polymer glasses (up to about 1940), buffers were commonly employed as pH modifiers and pH control agents during resin manufacture as taught by U.S. Pat. No. 1,737,918; 1,791,062; 1,952,598; 2,015,806; 2,647,212; and 2,729,616, and also more recently in U.S. Pat. No. 3,637,562 and 4,139,507. It must be realized that in the early part of this century, measurement of pH was difficult task, and therefore buffer systems were employed so that pH monitoring was not necessary. This is particularly important for UF reactions since the rates of reactions are critically dependent upon pH. In no instance are buffers referred to as cure catalysts, but rather, where applicable in these patents, it is found necessary that free acid or acid generating salts are employed to overcome buffers to achieve adequate cure. In no case is it suggested that the presence of such neutralized buffers result in enhanced cure speed of any sort. For example, U.S. Pat. No. 1,952,598 to Luther discloses buffers are inert materials, and take no part in the UF reaction process other than as pH control agents.
The use of buffering-type salts have also been employed in the use of UF resin for high nitrogen fertilizers. In these instances buffers are used either as a source of potassium and phosphorus for fertilizers (3,479,175) or for pH control during manufacture of the low condensed resin (4,781,749 and 4,895,983). Cure is not usually a consideration for fertilizer resins, but in U.S. Pat. No. 4,244,727 low levels of buffering are employed during manufacture of a UF resin fertilizer. This is analogous to the traditional requirements for minimal buffering to achieve cure in wood technology adhesive applications.
There are two reactions involved in reacting urea and formaldehyde resins: an "addition" reaction and a "condensation" reaction. These reactions are discussed in detail elsewhere in present specification. However, it is noted that buffer salts are known to accelerate the addition reaction between urea and formaldehyde to form methylolureas according to De Jong and De Jonge, Recueil Trav. Chim., 71, p.p. 643-660 (1952). The condensation reaction, which leads to polymer growth, high molecular weight and cure, has been explicitly reported to not be affected by buffering agents as taught by De Jong and De Jonge, Recueil Trav. Chim., 72, p.p. 139-156 (1953).